1. Field of the Invention
This invention relates to a lubricating oil composition having improved antiwear performance due to the presence of an alkyl alkoxyalkylxanthate and a metal thiophosphate.
2. Description of Related Art
Engine lubricating oils require the presence of additives to protect the engine from wear. For almost forty years, the principal antiwear additive for engine lubricating oils has been zinc dialkyldithiophosphate (ZDDP). However, ZDDP must be used in concentrations of 1.4 wt. % or greater to be effective. Since phosphates may result in the deactivation of emission control catalysts used in automotive exhaust systems, a reduction in the amount of phosphorus-containing additives (such as ZDDP) in the oil would be desirable. In addition, ZDDP alone does not provide the enhanced antiwear protection necessary in oils used to lubricate today's small, high performance engines.
Certain alkyl alkoxyalkylxanthanes are also known. For example, s-hexyl 0-(2-methyoxyethyl) ester, which has the formula: ##STR1## has a Chemical Abstracts Registry Number (RN=27098-36-8), but has not been used in lubricating oils. In addition, ethylene diisopropyl xanthate has been used in lubricants (see, for example, Russian Patent 655,717).
However, none of these publications suggest that the antiwear performance of a lubricating oil can be synergistically enhanced when certain alkyl xanthates and a metal thiophosphate are present therein.